Field of the Invention
The present invention relates to a rib or fin made of aluminum or an aluminum alloy for the joint ribbing of a plurality of heat exchanger tubes in a ribbed-(or finned) tube heat exchanger for motor vehicles. More particularly, the present invention relates to a rib or fin made of aluminum or an aluminum alloy for the joint ribbing or finning of a plurality of heat exchanger tubes in a ribbed or finned tube heat exchanger for motor vehicles, wherein: the ambient air flows, as a first heat exchange fluid, along the surface of the rib and a second heat exchange fluid is conducted in the heat exchanger tubes; the rib is corrugated in the direction of flow of the first fluid; connecting sleeves for connection to the heat exchanger tubes are shaped to the rib, with at least one corrugation wave crest extending between two connecting sleeves disposed adjacent one another transversely to the flow direction of the first fluid; and local air guidance profiles shaped in the corrugated surface of the rib are provided in the spaces between connecting sleeves.
Heat exchanger ribs of the above type are disclosed in DE-OS No. 2,530,064. They are charged from the exterior by the ambient air as the first heat exchange fluid while a second heat exchange fluid is conducted within the heat exchanger tubes which are ribbed or finned by the addition of ribs or fins.
At present, ribs or fins for ribbed tube heat exchangers in motor vehicles are generally manufactured of aluminum or aluminum alloys with very thin wall thicknesses between typically 0.08 and 0.15 mm. The manufacture of such ribs of sheet iron is practically out of the question because their thermal conductivity is four times worse than that of aluminum ribs, and for reasons of corrosion and weight. High-grade steel sheet would be resistant to corrosion but has only about 10% of the thermal conductivity of an aluminum rib or fin. The manufacture of such ribs of copper would meet the requirements with respect to corrosion resistance or thermal conductivity, which is even better, but, except for some special cases, e.g. in some engine radiators or solderable heating system heat exchangers, cannot be used for reason of their weight and because of the price of copper compared to the price of aluminum.
In this sense, the development of heat exchangers for motor vehicles as mass produced articles has been geared toward optimizing not only the performance data, but also their weight, structural volume, the use of material and the like, so as no longer to permit a simple comparison with heat exchangers for other applications which are produced in small numbers down to production in individual units.
For such ribs or fins it is desired to produce, with easily manufactured, durable means, the highest possible heat transfer coefficient between the rib or fin, on the one hand, and the gaseous first fluid charging the rib, on the other hand. This increase in the heat transfer value brings about savings in investment costs and during operation since, with the same quantity of heat to be transferred and at the same operating temperatures, the frontal surface of the heat exchanger (the upstream surface) and the structural depth can be reduced or the spacing of the heat exchanger ribs can be enlarged. Since the heat exchanger rib or fin is used in ribbed-tube heat exchangers in motor vehicles, the reduction of structural volume and the concomitant reduction in the weight of the heat exchanger are of decisive significance. This applies to their possible use in motor vehicle radiators or as heating system heat exchangers, as well as to their preferred use in liquefiers or evaporators in motor vehicle air-conditioning systems.
The exchange of heat between the two fluids is effected by means of heat radiation, heat conduction and convection, particularly, however, by way of convection in which the heat is transferred by moving particles of a substance. The exchange of heat by convection is decisively dependent on the type of flow of the first gaseous fluid around the tubes and the heat exchanger rib or fin.
It is known that with a flow parallel to a plate, a laminar boundary layer forms at the surface and becomes thicker with increasing length of the flow path L to thus impair the exchange of heat by convection since this laminar boundary layer is able to transfer the heat only by way of molecular conduction processes. Qualitatively, the external heat transfer coefficient is described by the following formula: ##EQU1##
where .alpha..sub.a is the heat transfer coefficient averaged over the length L of the plate for heat transfer from the gaseous fluid to the surface of the rib, fin or plate;
w is the flow velocity of the gaseous medium charging the ribs;
c is a constant resulting from the physical characteristics of the flowing medium.
The equation indicates that the heat transfer coefficient of plates can be improved by either increasing the flow velocity or decreasing the length of plate L over which the fluid flows.
A further reduction in the exchange of heat is the result of dead flow spaces which build up downstream of the tube regions when seen in the direction of flow of the first fluid, i.e. in their areas shaded from the flow. Due to a low intensity stationary turbulence created by the flow downstream of the heat exchanger tubes, the local heat transfer coefficients at such regions become considerably smaller than in the regions in the path of the main stream. With continued reduction of the rib, fin, or plate thickness, the heat conduction resistance in the rib must be given increasing consideration, and this results in the requirement for the most uniform heat current density throughout the rib with constant tube spacing. This, in turn, is realized by adapting the local heat transfer coefficients. To meet this requirement, it is known to give the ribs various profiles. One of the simplest known profiles resides in a corrugated configuration of the rib in the direction of flow of the first fluid so that the wave crests and troughs extend transversely to this direction of flow (see, for example, DE-OS No. 2,530,064 which belongs to the same species, as well as for example DE-OS No. 2,756,941). This corrugation, on the one hand, slightly lengthens the flow path and thus the flow velocity between the ribs or fins and, on the other hand, the required deflection of the air in the corrugated rib causes the laminar boundary layer to be at least partially reconstructed after each wave crest, thus avoiding, at least somewhat, an enlargement of the boundary layer and a reduction of the external heat transfer coefficient corresponding to the above equation. This is particularly applicable if the wave crests are relatively sharp, particularly if they have the shape of edges of a linear zigzag corrugation. However, rounded wave crests are also included in the scope of the present invention.
Yet, there are limits for the external heat transfer coefficient with respect to a smooth plate since, beginning with a corrugation angle .theta. of 15.degree. to 20.degree.,the heat transfer coefficient increases only slightly while the pressure loss on the air side increases more and more. The increase in the external heat transfer coefficient realized exclusively by corrugation is insufficient in the range of an industrially worthwhile ratio of power increase to increased pressure losses, i.e. corrugation angles up to a maximum of 20.degree., since the reduction of the laminar boundary layer by the corrugation is insufficient and, moreover, the increase in surface area as well as the increase in flow velocity are still relatively small (6%) for a corrugation angle of at most 20.degree.. Additionally, corrugated rib surfaces do not result in a significant reduction of the dead flow spaces downstream of the tubes and in an optimum distribution of the local heat transfer coefficients with respect to uniform radial heat current density. Corrugation angles of about 45.degree., as they are shown in the drawings of DE-OS No. 2,530,064, lead to a larger boundary layer through which the heat must be transported as a result of molecular conduction of the air since the air flows only to a small extent parallel to the corrugation and a low intensity stationary turbulence develops in each wave trough. The great increase in surface area of 30% to 40% depending on the surface area percentage of the sleeve-shaped tube connection areas is compensated for the most part by the above-described increase in the thickness of the boundary layer so that the increase in the external heat transfer coefficient, as experience has shown, is only insignificantly larger than for a corrugation angle of 20.degree. (FIG. 7).
For uncorrugated ribs or fins, new profile shapes have been developed which are pressed out of the rib itself, thus producing perforations in the rib material. All these profiles have in common that the shaping of the rib is to prevent, as much as possible, the development of a laminar boundary layer of greater thickness. A prior art profile of this type (DE-GM No. 78 06 410) attempts to conduct the gaseous fluid in a pressed-out guide channel which has a semicircular cross section into the flow shade area behind the tube connection locations. The cutting edges created at the beginning and end of each guide channel require a new development of the thermal and hydraulic flow profile. Aside from the fact that it is doubtful that the guide channels interfere with the formation of performance reducing turbulence regions, the cutting edges are limited to only a small percentage of the surface area of the rib, while a large portion of the surface area of the rib or fin is configured as a smooth rib or fin without boundary layer reducing profiles.
A more uniform distribution of rib perforations and guide webs is disclosed in the prior art arrangement (DE-OS No. 2,5128,226) of a heat exchanger rib or fin for the chemical industry, particularly the petroleum industry, in which a plurality of narrow guide webs, with which the laminar boundary layer is to be forced to constantly reconstruct itself, are provided between two tube connections in the rib. However, in fact, this measure appears to be a drawback since the slits in the heat exchanger rib or fin associated with the outer guide webs would then make the flow of heat from the tubes, to be attached at the tube connections, to the outer guide webs more difficult because of a partially considerably longer flow path, so that a greater temperature difference would be required for the heat transport and thus the efficiency of the fins would be reduced. Since the outer guide webs are disposed closely on top of one another in the same plane, the boundary layer formed in the preceding slit is also not reduced completely. Moreover, the automotive engineer of average skill in the art, even if he is occupied with the construction of heat exchangers for motor vehicles and particularly their air-conditioning systems, will not look around among heat exchanger structures for the chemical industry.
The last mentioned technical drawback is overcome in DE-OS No. 3,131,737 which relates to room heating and cooling systems in that the ribs or fins of the ribbed tube heat exchangers disclosed there have guide webs in the form of embossed roof-shaped strips which are arranged to form bridges with respect to one another. Although this configuration of the guide webs is better with respect to boundary layer reduction and rib stability, it again has the drawback that it is impossible to develop a radial heat flow with an at least approximately invariable angle around the heat exchanger tubes.
Moreover, the manufacturing tools for this rib or fin are particularly complicated and, for a given performance, high pressure losses result, particularly in the case of additional condensation of steam due to the rib temperature dropping below the dew point. In the latter case, condensation water will be retained between the many guide webs by adhesion as in a sponge, so that the rib or fin surface is blocked with condensation water and the heat transfer becomes even worse than with a smooth fin.
According to an improvement (German Patent No. 3,336,985) of this basic type of construction, air-side pressure losses are reduced with the heat transfer coefficient remaining the same and the required manufacturing tools are simplified in that at least one perforation is provided between adjacent tube connections in the same row and at least one guide web is provided for the gaseous fluid, with this guide web being placed out of the plane of the rib or fin at an edge of the perforation which extends transversely to the row and adjacent a tube connection location. The guide web, on the one hand, reduces the laminar boundary layer and, on the other hand, conducts the air in such a manner that the formation of a turbulence region downstream of the tubes is avoided. However, the required width of the guide webs, measured in the direction of flow of the gaseous fluid, of at least three-quarters of the outer diameter of the tube connection, reduces the stability of the ribs or fins to such an extent that, with a given rib (fin) stability, the thickness of the material must be increased, as the use of harder rib material is made impossible due to the maximum attainable height of up to 2.4 mm for the sleeve-type tube connections. Even if the thickness of the material were increased from 0.12 mm to 0.15 mm, certain stability problems would result in handling during the manufacturing process in a packet of ribs in which the tubes are not yet introduced into the sleeve-shaped tube connections so that relatively high production times must be accepted. Moreover, the problems of soiling and the entrapment of water if the rib temperature drops below the dew point are not completely solved, analogously to the heat exchanger rib or fin according to DE-OS No. 3,131,737.
In the same type of rib or fin disclosed in DE-OS No. 2,530 064 intended for use in motor vehicle radiators, attempts have already been made to further improve the heat transfer coefficient of a corrugated fin in that flaps projecting from the fin and formed of punched-out tear holes serve to form spacers between successive individual ribs in the rib packet of the ribbed tube heat exchanger and are set at an angle to the flow of the first fluid to serve as local air guide profiles. By a grid-like arrangement of these sloped air guide sheets in the gaps between mutually offset heat exchanger tubes or between the connecting sleeves of the rib accommodating the heat exchanger pipes, the charging of the rib or fin with the first fluid in the regions shaded from the air by the heat exchanger tubes is to be improved. The corrugation of the rib or fin is here so short-waved that each tear hole together with two mutually parallel flaps occupies two slopes of the corrugation while bridging a wave crest in each case. In the region of the tear hole, the corrugation and its desirable effect are then eliminated. However, this configuration including the tear holes and the sharply bent flaps which take up the entire space between adjacent ribs is predestined to collect condensation water generated when the rib temperature drops below the dew point as well as dirt. Moreover, the flaps which take up the entire space between ribs are relatively large-area turbulence generators with their own air shaded region effect for the gaseous first fluid and, connected with this, even cause a reduction of the heat transfer between the ribs and the gaseous first fluid. The oblique position of the flaps in only one possible sloping direction appears to be unmotivated since the first fluid does not know a preferred lateral direction of flow. Multiplying the flaps serving as spacers would increasingly block the flow cross section of the rib or fin packet for the first fluid and would thus likewise counteract the desired increase in efficiency. The resulting improvement of the prior art measure with respect to the heat transfer coefficient should therefore at most be slight and would become significant only if spacers for the ribs or fins in the rib or fin packet were separated from the holding sleeves for the ribs or fins. Instead, within the scope of the present invention, such separate spacers are preferably avoided entirely since applicant's earlier rib or fin structures (see German Patent No. 3,336,985) already provide the connecting sleeves of the ribs, where they are attached to the heat exchanger tubes, with collars which engage in corresponding grooves on the rear of the next fin and thus permit use of the connecting sleeves simultaneously as spacers for the fin packet. However, the present invention is not limited to this case but also leaves open the use of separate spacers, although this is not a preferred possibility.
Essentially closed bulges made in planar ribs or fins are known for ribbed-tube heat exchangers which are disclosed for other materials than for aluminum or aluminum alloys. For example, this is disclosed in German Patent No. 496,733 which dates from 1930 and in which the ribs or fins are made of sheet metal and are soldered to the tubes. Obviously, sheet iron or stainless steel (high-grade steel) sheet material is contemplated since the configuration of sheets with choke locations disposed between the tubes and air guide means for guiding the air flowing through the fin packet into the shaded air flow regions behind the tubes is directed to a material in which otherwise the lack of thermal conductivity in the above-mentioned shaded air flow regions would result in excess temperature drops and thus in a reduction of efficiency. This would not be the case with ribs or fins made of copper. Thin ribs or fins made of aluminum in which air flow shaded region problems also result did not exist at that time. Moreover, the expressly desired throttling between the tubes leads to high pressure losses. For all of these reasons, this prior art heat exchanger is not suitable for use in motor vehicle construction.
Particularly for motor vehicle radiators, solderable ribs or fins, primarily made of copper, are already disclosed in U.S. Pat. No. 1,575,864 of 1926 in which pointed, particularly conical, bulges which are closed in the plane of the rib (fin) project on one or both sides from a planar rib. Such bulging in planar fins have been considered again and again since the thirties, even by applicant, in various modifications, but has just as often been rejected because the realizable increase in surface area and the initiation of turbulence is not sufficient for the required performance density compared to other disclosed configurations of that time and of the type discussed above. Moreover, this prior publication does not provide an example for possibly arranging such bulges in such a manner that the flow is conducted into air shaded regions downstream of the tubes. Because of the use of copper as the rib material, this is also not necessary in the prior art heat exchanger.
Ever since the early thirties, development of rib or fin constructions for ribbed- (finned-) tube heat exchangers for motor vehicles has gone different ways, without it being considered to combine corrugated ribs or fins and essentially or completely closed bulges on ribs in some way. To the contrary, in the above-mentioned German Patent No. 496,733, the throttling structure selected there is expressly mentioned as an alternative for a certain type of known corrugation, namely a corrugation concentrically around the tubes, without considering at all a combination of corrugation and bulges.